Expanding the text here will generate a large
amount of data for your browser to display

4. The maintenance requirement (MR)

This is a subject on which there is still a difference of opinion.
Clearly when growth is rapid the IAA requirement must to a large extent be
determined by the pattern of IAAs in the tissue that is being laid down. In the
non-growing adult the MR derives from the need to replace IAAs that are consumed
in a variety of irreversible pathways (Table 3). There is no a priori
reason why this consumption pattern should bear any relation to that of
deposition; a view that has been widely held by nutritionists in the past, e.g.
Osborne & Mendel (1916, quoted by Millward, 1992) and Said & Hegsted
(1970), and which is accepted in principle by Young (personal communication).
Thus Fuller et al (1989), in experiments on young pigs, have assessed separately
the pattern of requirements for growth and the pattern when there is no growth
(maintenance), and found the two patterns to be quite different.

Table 3 Some non-protein pathways of amino acid utilization

Amino acid

Pathway

Methionine

Methylation reactions

Creatine

Choline

Cysteine

Glutathione

Taurine

Tyrosine

Neurotransmitters

Glutamate

Neurotransmitters

Lysine

Carnitine

Glycine

Nucleic acid bases

Haem

Creatine

From Reeds (1990).

Nevertheless, Young and El Khoury (1995) maintain that in practice
in man the maintenance IAA pattern resembles that of tissue protein, on the
grounds that there is a close correspondence between the tissue pattern and that
of the IAA requirements of pre-school children, as observed by the workers at
INCAP (Pineda et al, 1981) (Table 1B). Since in the pre-school child
growth represents only some 10% of the requirement, it is reasonable to
extrapolate these results to adults.

Is it possible to reconcile these two opposing points of view? On
a protein-free diet or fasting the obligatory N loss (ONL) must reflect the
pattern of tissue proteins, since the only source of nitrogen is from protein
breakdown. In this situation one may suppose that the IAA with the largest
consumption pathway 'drives' the obligatory loss. If protein breakdown provides
more of a particular IAA than is needed to make good its loss through
irreversible pathways, this extra amount must nevertheless be oxidized. When
protein is fed, the pattern changes and the IAAs are only needed in a pattern
that balances their losses in the consumption pathways. Thus if fasting and
feeding occur in a 12 h cycle, 50% of the 24 h requirement will reflect the
composition of body protein and 50% that of the consumption pathways.

The picture, however, is complicated by the diurnal cycling of
deposition and loss of body protein (see below). Fed state deposition can be
regarded as a form of temporary growth, and therefore requires that amino acids
should be provided in the concentrations in which they occur in body protein,
although Fuller (personal communication) has suggested that temporary protein
storage could have a composition quite different from that of body protein as a
whole. To a large extent these amino acids must be derived from protein
breakdown; if the food intake is, say, one fifth of the flux, four fifths of the
amino acids deposited will be derived from breakdown. At maintenance levels of
intake, deposition is relatively small, but at higher intakes, when deposition
is increased, the rate of protein breakdown is greatly reduced (section 11), so
that intake from the diet becomes more important. Millward et al (1991)
have discussed whether amino acids liberated from body protein during the
fasting period could be held over, as it were, and be available for meeting
needs in the fed state. However, the free amino acid pools, particularly those
of the branched chain amino acids (BCAs), are too small for this to be likely.
Therefore, in the fed state, particularly with generous intakes, the intake from
food plays an essential role in topping up the amino acid supply for protein
deposition, and to the extent that it is used in this way, this intake probably
must have the pattern of tissue protein.

It follows that conceptually the IAA requirement pattern will be
some kind of halfway house between the pattern of irreversible losses and the
pattern of body protein, and the relative proportions of the two patterns will
be influenced by the level of protein intake. It seems to me that the next step
must be to measure the individual IAA losses through irreversible pathways. In
the meantime, because the two processes of fasting loss and diurnal cycling tend
to shift the maintenance pattern towards that of body protein, it may be
considered that Young's proposal represents a reasonable working compromise
until more direct observations become available. It should be noted that on
Young's hypothesis, if the requirement of one IAA is established, those of all
the others follow, whereas if an amino acid, e.g. Lysine, is differentially
conserved, the requirement of each IAA must be determined
separately.